Differences between M50 and M50x are very subtle. It's the same driver, the same housing, the same tuning, and only different earpads. I commented in the past that M50x bass was a bit more detailed and staging a bit wider, but I think it was all due to thicker/plushier pads and a "new toy" syndrome lol!!! If your M50 is broken and you need a replacement, I would obviously recommend going with M50x instead of M50. But if you want a different sound sig (based on reading here, higher quality and lower quantity bass and more open brighter/detailed upper mids/treble) - MSR7 might be a good way to go. Just keep in mind, M50x is more of a replacement (with RC convenience) rather than upgrade from M50.
Oh, and forgot to mention "welcome to head-fi, sorry about your wallet"